Fuel cells have attracted attention in recent years in consideration of environmental issues and energy issues. Fuel cells generate electrical power by a reverse reaction of the electrolysis of water that uses hydrogen and oxygen, and provide a clean electrical power generation device for which the only waste product is water. Although fuel cells are classified into several types corresponding to the type of electrolyte used, since polymer electrolyte fuel cells in particular operate at a low temperature, they are the most promising for use in automobiles and consumer applications. Such fuel cells are able to achieve high output power generation by laminating single cells composed of, for example, a solid polymer electrolyte, gas diffusion electrode, catalyst and separator.
In a fuel cell having the aforementioned configuration, flow paths (grooves) for supplying fuel gas (such as hydrogen) and oxidant gas (such as oxygen) and discharging generated moisture (water vapor) are normally formed in separators for partitioning single cells. For this reason, separators are required to have high gas impermeability that enables complete separation of these gases as well as high electrical conductivity for reducing internal resistance of the flow path portion. Moreover, they are also required to have superior thermal conductivity, durability, strength and the like.
Since the fuel cells used in automobiles in particular are required to have large capacity, in addition to the aforementioned characteristics, these fuel cells are also required to be free of the occurrence of cracking and breakage caused by vibrations during vehicle operation despite being thin and having a large surface area.
Fuel cell separators are broadly classified into metal separators, carbon sheet separators and separators formed by mixing a resin into a carbonaceous material. Among these, separators obtained by mixing a resin into a carbonaceous material offer the advantage of allowing separators that are more resistant to corrosion than metal separators and more resistant to bending and vibrations than carbon sheets to be obtained at low cost.
Although it is necessary to fill fuel cell separators obtained by mixing a resin into a carbonaceous material with a large amount of carbon material in order to ensure adequate electrical conductivity, when these separators are filled with a large amount of carbon material, the separator becomes brittle which tends to result in increased susceptibility to cracking and breakage.
Although efforts to improve mechanical characteristics by reinforcing resin moldings with carbon fiber have been made in the past in order to prevent cracking and breakage of resin moldings, since moldability becomes inferior and dimensional precision of the molding tends to become poor as the amount of carbon fiber incorporated in the resin molding increases, carbon fiber is normally only added in small amounts, and it difficult to obtain a separator that is favorable with respect to all requirements relating to mechanical characteristics, electrical conductivity and dimensional precision simply using carbon fiber for the carbon material used in separators. Consequently, various contrivances have been made to obtain a separator that is favorable in all aspects of mechanical characteristics, electrical conductivity and dimensional precision.
For example, Patent Document 1 and Patent Document 2 disclose a fuel cell separator obtained by molding a resin composition containing graphite and carbon fiber.
In addition, Patent Document 3 discloses a fuel cell separator obtained by molding electrically conductive fiber sheets, having carbon fibers arranged in the same direction, interposed about an electrically conductive resin sheet containing another electrically conductive substance such as graphite.